Semiconductor device and manufacturing the same

ABSTRACT

A semiconductor device is disclosed in which a heat sink is difficult to warp and which is inexpensive. The semiconductor device comprises a heat sink, a pair of screwing pieces having respective inner end portions connected to both ends of the heat sink and further having outer end portions respectively formed with through spaces for screwing, one or more semiconductor chips fixed to a main surface of the heat sink, a seal member formed of an insulating resin which covers a back side of the heat sink and also covers the other portions of the paired screwing pieces than the outer end portions, a first lead having an outer end portion projecting from the seal member and further having an inner end portion which is positioned within the seal member and whose inner end is arranged near one side face of the heat sink, a second lead having an outer end portion projecting from the seal member and further having an inner end portion positioned within the seal member and near another side face of the heat sink, and plural conductive wires for electrically connecting between the leads and predetermined electrodes of a predetermined semiconductor chip, or between the leads and predetermined electrodes of a predetermined semiconductor chip and also between predetermined electrodes of a predetermined semiconductor chip and predetermined electrodes of a predetermined semiconductor chip.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 10/318,018filed Dec. 13, 2002.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor device and a method ofmanufacturing the same. Particularly, the invention is concerned with atechnique which is effectively applicable to a manufacturing techniquefor a high frequency power amplifier (high frequency power amplifiermodule) for a base station in a radio communication system such as acellular communication system.

SUMMARY OF THE INVENTION

In a radio communication system such as a cellular communication system,a portable telephone set (a portable terminal) is connected to a basestation adjacent to a telephone network by a speaker's operation of thetelephone set, then is connected successively to a single or plural basestations, and finally a portable terminal of the party being called iscalled (originating a call), thereafter the portable telephone setassumes a state which permits talking with the called party. In thiscase, the base station amplifies the received signal and transfers thethus-amplified signal. Such an amplification is performed by means of ahigh frequency power amplifier for a base station.

Having made studies about the cost reduction of a high frequency poweramplifier for a base station, the present inventor reviewed a sealingstructure (package structure) as a main cause of high cost.

FIGS. 28 to 39 illustrate a semiconductor device (hereinafter referredto as the “studied semiconductor device”) which the present inventor hadstudied prior to the present invention. The studied semiconductor deviceis a high frequency power amplifier (a high frequency power amplifiermodule). FIG. 28 is a plan view of the studied semiconductor device,FIG. 29 is a side view thereof, FIG. 30 is a plan view thereof with acap removed, and FIG. 31 is a schematic sectional view of the studiedsemiconductor device shown in FIG. 30.

A semiconductor device 70 is manufactured using a rectangular substrate71. The substrate 71 is formed of a metal (e.g., CuMo plate) which issuperior in thermal conductivity so that heat generated from asemiconductor chip is transferred promptly to an installed radiationboard or mounting substrate, thus serving also as a heat sink.

A quadrangular frame 72 is fixed by bonding (silver soldering) centrallyonto the substrate, or heat sink, 71, the frame 72 having a widthsmaller than the width of the heat sink 71 and having a length shorterthan long sides of the heat sink 71. The heat sink 71 serves as a sourceelectrode. Screwing grooves 73 are formed centrally of both ends of theheat sink 71 at positions spaced apart from the frame 72.

As shown in FIG. 31, the frame 72 comprises a frame-shaped ceramic base74 and two frame-shaped ceramic sleeves 75 superimposed successively onthe ceramic base 74. A metallized surface layer 75 a (the dotted regionin FIGS. 28 and 30) is formed on the surface of the upper ceramic sleeve75.

Along the width of the heat sink 71 the ceramic sleeves 75 aresuperimposed one on the other at the same width as the width of theceramic base 74 so that their inner and outer wall surfaces are inregistration with the ceramic base, while in the long sides of the heatsink 71 the ceramic sleeves 75 are smaller in width than the ceramicbase 74 and the surface of the ceramic base is exposed to both insideand outside of the ceramic sleeves 75.

On the surface of the ceramic base 74 extending along the long sides ofthe heat sink 71 there is formed an electrically conductive metallizedlayer 76. Consequently, the metallized layer 76 is exposed to thesurface portions of the ceramic base 74 inside and outside the ceramicsleeves 75. The metallized layer portions located inside the ceramicsleeves 75 serve as bonding posts 76 a for wire connection, while themetallized layer portions located outside the ceramic sleeves 75 serveas lead connecting portions 76 b for lead connection.

An inner end of a drain lead 77 formed of a broad metallic plate isfixed to the lead connecting portion 76 b extending along one long sideof the heat sink 71, and an inner end of a gate lead 78 formed of abroad metallic plate is fixed to the lead connecting portion 76 bextending along the other long side of the heat sink 71. One corner ofthe drain lead 77 is cut off obliquely, serving as an electrode index 77a for recognition of a drain electrode (see FIG. 28). The metallicplates which constitute the drain lead 77 and the gate lead 78 areformed of Kovar or Fe—Ni alloy, having such characteristics as thermalexpansion coefficient and thermal expansion coefficient difference ofceramic being small and has a resistance to a high temperature in silversoldering.

On an upper surface of the heat sink 71 located inside the frame 72there are fixed a capacitor chip 79, an SiMOSFET chip 80, and acapacitor chip 81 side by side from the drain lead 77 toward the gatelead 78. In the capacitor chips 79 and 81, an oxide film (SiO₂ film) isformed on a silicon substrate and an electrode is formed thereon toafford a predetermined capacitance. The silicon substrate serves as oneelectrode, while the electrode on the oxide film serves as the otherelectrode. These chips are fixed by AuSi eutectic, which is satisfactoryin heat radiating property and electric conductivity and high inreliability, to the heat sink 71 formed of a metallic plate, so thatthere is ensured an electric contact between the substrate-sideelectrode of each chip and the heat sink 71 and there is obtained asatisfactory heat radiating property.

As shown in FIG. 30, one end of each conductive wire 82 is connected toa drain electrode 80 d on the SiMOSFET chip 80, while an opposite endthereof is connected to an upper electrode 79 a of the capacitor chip 79at a position close to the drain lead 77. Likewise, one end of eachconductive wire 83 is connected to the upper electrode 79 a of thecapacitor chip 79, while an opposite end thereof is connected to thebonding post 76 a which is connected electrically to the drain lead 77.Further, one end of each conductive wire 84 is connected to one end of agate electrode 80 g on the SiMOSFET 80, while an opposite end thereof isconnected to an upper electrode 81 a of the capacitor chip 81 locatedclose to the gate lead 78. One end of each conductive wire 85 isconnected to the upper electrode 81a of the capacitor chip 81, while anopposite end thereof is connected to the bonding post 76 a which iselectrically connected to the gate lead 78.

The wires 81 to 85 are formed of aluminum wires. As to the wiresconnected to drain and gate electrodes, there is adopted a structurewherein plural wires are connected in parallel to ensure currentcapacity and high frequency characteristic (see FIG. 30).

As shown in FIG. 28, a cap 87 is fixed to the frame 72. The cap 87 isformed of a ceramic plate. As shown in FIG. 29, in a peripheral portionon a back side of the cap 87, a plating film 87 a is formed of AuSn in aframe shape correspondingly to the ceramic sleeves 75, and the ceramicsleeves 75 are fixed through the plating film 87 a. That is, the cap 87is fixed to the frame 72 hermetically (hermetic seal structure) throughAuSn alloy.

Along a side face of the frame 72 there is formed a side metallizedlayer 76 d, as indicated with dots in FIG. 29. The side metallized layer76 d provides an electric connection between the heat sink 71 serving asa source electrode and the surface metallized layer 75 a, placing thesurface metallized layer 75 a at an equal potential to the sourceelectrode.

Exposed portions of the heat sink 71, drain lead 77, gate lead 78,surface metallized layer 75 a, side metallized layer 7 d, and metallizedlayer 76 are wholly plated with Au.

It is difficult to reduce the manufacturing cost of such a semiconductordevice. More particularly, (1) a heat sink made of CuMo is closelysimilar in thermal expansion coefficient to a semiconductor chip formedof a ceramic material or Si and exhibits a package stress diminishingeffect; besides, it is superior in heat radiating property, so itbecomes possible to ensure a stable operation of the associatedsemiconductor device. However, the material cost is high and machiningis troublesome, leading to an increase of cost. In the structure of thesemiconductor device, moreover, the size of the heat sink used is largeand hence an increase of cost results.

(2) Thick gold plating is needed for ensuring a required heat resistancein AuSi eutectic chip bonding, but a selective gold plating is difficultand there must be adopted a whole-surface plating method, withconsequent increase of cost. It is necessary that the gold plating beperformed to a thickness sufficient to ensure such a heat resistance asprevents the occurrence of discoloration of plating due to heating inchip bonding. For AuSi eutectic, in which heating is made to about 430°C., a plating thickness of 2 μm or more is needed, constituting one ofcauses of an increased cost.

(3) AuSn sealing work costs high. Besides, there is obtained a packageof a hollow structure and a hermetic seal structure, so it becomesnecessary to conduct a hermetic seal leak inspection and a movable dustparticle inspection (dust particle inclusion inspection) as additionalworks, thus resulting in that the inspection cost further increases.

In more particular terms, since reliability depends on whether hermeticseal performance is good or not, it is necessary to conduct a hermeticseal leak inspection, which causes an increase of the assembling cost.Moreover, the management of oxygen control is important for ensuring ahigh hermetic seal performance, which causes deterioration of thesealing work efficiency and hence an increase of the assembling cost.

Further, if a hollow package is sealed with electrically conductive dustparticles mixed therein, the dust particles move around the interior ofthe package under vibrations imparted to the package from the exterior.If the moving dust particles span the electrodes, there is a fear thatan electrically semi-shorting defect may result, thus requiring the dustparticle inclusion inspection as an essential condition.

On the other hand, a review of the semiconductor device 70 from thestandpoint of electrical characteristics has revealed that the followingproblems are involved therein.

In the case of a base station for mobile telephone, the frequency isabout 0.8 to about 2.1 GHz and an output as large as 60 to 250 W isrequired as the output of a final-stage power amplifying FET. A supplyvoltage of an amplifier which uses MOSFET (Metal Oxide SemiconductorField-Effect-Transistor) made of Si as FET for output is about 28V, soin the case of an FET with an output of 125 W and a drain efficiency ofabout 50%, an average current is about 9 A and a peak current reaches 27A which is about three times as large as the average current.

Since such a large current is handled, it is necessary to use alarge-sized FET chip with a gate width of 20 cm or so, and input andoutput capacitances become as large as nearly 150 PF and 80 PF,respectively, resulting in the device being very low in impedanceACwise. For this reason, a high frequency loss increases at a frequencyband of 1.5 GHz or higher. For avoiding this inconvenience, an internalmatching circuit is included between FET chip and package electrodeleads to increase the impedance of the lead portions.

FIG. 32 is an equivalent circuit diagram in which internal matchingcircuits are provided on an input side (gate side) and an output side(drain side), respectively. Between a gate electrode 80 g of SiMOSFETchip 80 and an input terminal Pin is formed an input matching circuit byinductances L1, L2 of wires 85, 84, capacitance C1 of the capacitor chip81, and input capacitance Ciss of SiMOSFET chip 80.

Further, between a drain electrode 80 d of SiMOSFET chip 80 and anoutput terminal Pout is formed an output matching circuit by inductancesL3, L4 of wires 82, 83, capacitance C2 of the capacitor chip 79, andoutput capacitance Coss of SiMOSFET 80. A source electrode 80 s for thedrain electrode 80 d, as well as one terminals of the capacitors, areconnected to ground (GND).

With the input matching circuit, an impedance of several ten ohms isconverted to a low impedance of 1 ohm or less, while with the outputmatching circuit, an impedance of 1 ohm or less is converted to animpedance of several ten ohms, thus permitting an efficient poweramplification.

The wires 82 to 85 serve as appropriate inductances for a desiredfrequency characteristic and the wire length (wire loop shape) isdesigned so as to form an internal matching circuit.

However, for forming the internal matching circuit it becomes necessaryto ensure an additional space for the capacitor chips 79 and 81, whichis disadvantageous to the needs for the reduction of size, and it isalso necessary to perform a wire bonding work many times in a uniformloop shape, which causes a deteriorated yield in assembly.

Now, with reference to FIG. 33, a description will be given about asectional structure of SiMOSFET 80 and a current path thereof. FIG. 33illustrates a section and current path of a horizontal type SiMOSFETchip used for high frequency power amplification.

The horizontal SiMOSFET has a p⁺ silicon substrate 90 of a highconcentration and a p⁻ epitaxial layer 91 of a low concentration whichis formed on the p⁺ silicon substrate 90 for obtaining a requiredvoltage proof. In a surface portion of the p⁻ epitaxial layer 91 areformed a channel-forming p layer 92 and a drain n⁺ layer 93 of a highconcentration which is located at a position spaced a predetermineddistance from the channel-forming p layer 92. Further, an n⁻ drainoffset layer 94 of a low concentration is formed in the surface layerportion of the p⁻ epitaxial layer 91 from the drain n⁺ layer 93 up tothe channel forming p layer 92.

The channel-forming p layer 92 is formed relatively deep and asource-forming n⁺ layer 95 of a high concentration is formed on theleft-hand side of a surface portion of the channel-forming p layer 92.The region from the left end of the source-forming high concentration n⁺layer 95 up to the pn junction formed of both channel-forming p layer 92and n⁻ drain offset layer 94 serves as a channel-forming region.

A p⁺ through diffusion layer 96 of a high concentration is formedthroughout a depth extending from the left end of the source-forminghigh concentration n⁺ layer 95, past the p⁻ epitaxial layer 91, up to asurface portion of the p⁺ silicon substrate 90. An insulating film(oxide film 97 is formed selectively on the surface of the p⁻ epitaxiallayer 91. The insulating film 97 extends from an intermediate positionof the source-forming high concentration n⁺ layer 95 up to anintermediate position of the drain n⁺ layer 93. At the right end of theinsulating film 97 a drain electrode 80 d is formed on the drain n⁺layer 93, and at the left end of the insulating film 97 a source wiringline 80 sa is formed from the source-forming high concentration n⁺ layer95 onto the p⁺ through diffusion layer 96.

At a position deviated from the channel forming region, a source fieldplate 98 is provided on the insulating film 97 which overlies the n⁻drain offset layer 94. The source field plate 98 is fixed at a sourcepotential and is improved in drain voltage proof by relaxing an electricfield.

The insulating film 97 is thinned at its portion opposed to thechannel-forming region, which thinned portion serves as a gateinsulating film (oxide film) 97 a. A gate electrode 80 g is formed onthe gate insulating film 97 a and is electrically connected to a gatewiring line 80 ga which extends onto the insulating film 97.

By a high accuracy by both ion implantation technique and diffuisontechnique, p- and n-type diffusion layers (regions) are formed, wherebyan n-channel enhancement type MOSFET is formed.

In such a device structure, a thick-line arrow shown in FIG. 3represents a current path, along which an electric current 99 flowsthrough the drain electrode 80 d, drain n⁺ layer 93, n⁻ drain offsetlayer 94, channel-forming p layer 92, source-forming high concentrationn⁺ layer 95, source wiring line 80 sa, p⁺ through diffusion layer 96,and p⁺ silicon substrate 90, and reaches the source electrode 80 s.

Current control is made by increasing or decreasing the gate potential,but since the horizontal FET is an enhancement type, it has a structuresuch that the drain current increases with an increase in gate potentialand is cut off as the gate potential approaches 0V. Thus, there accruesan advantage that a minus power supply for gate bias which is necessaryfor a depletion type GaAsFET can be omitted and that therefore it iseasy to implement a set circuit configuration.

The conventional high frequency power amplification using the horizontalFET is approximately class B amplification, in which a power componentbased on ON/OFF switching operation of a specified frequency isaccumulated in a tank circuit constituted by both inductor and capacitorand a high frequency power is taken out by a matching circuit.Therefore, the higher the drain voltage and the larger the drain currentcapable of being flowed, the higher the output obtained in FETconcerned, but for following up a high frequency operation it isnecessary that the capacitance be small and mutual inductance (Gm) belarge, so there is made such a chip design as affords high voltageproof, low capacitance, and high Gm. For attaining low capacitance andhigh Gm of FET chip, microminiaturization of gate length is necessaryand is now a trend in FETs for high frequency amplification.

As a great factor of obstructing the attainment of high Gm there ismentioned source resistance. If an apparent Gm observed in an actual FETis assumed to be Gm′ (exteriorly measurable Gm), Gm′ can be representedby the following relationship between true Gm (=Gmint) and sourceresistance (=Rs):

Gm′=(Gmint)/(1+Rs×Gmint)  (1)

Thus, for improving the performance of FET, the resistance diminishingprocessings subsequent to the formation of source n layer(source-forming high concentration n⁺ layer 95) shown in terms of theforegoing current path are important, and the decrease of contactresistance between the source electrode (=Si substrate) and the heatsink is an important subject in the assembling process. Moreparticularly, the AuSi eutectic chip bonding method is suitable,including the point of high heat radiating performance, and is appliedwhile imposing such a condition as does not develop voids in AuSieutectic.

Now, with reference to FIGS. 34 and 35, a description will be givenbelow about problems involved in space margin which are an obstructingfactor against the reduction in size of a heat sink as an expensivecomponent.

FIG. 34 is a side view as seen from a side face of the package forexplaining a space margin which is necessary for chip bonding of theSiMOSFET 80. For chip bonding, the SiMOSFET chip 80 is chucked andconveyed by a chip bonding collet 100 and is pushed and scrubbed againsta predetermined portion of the heat sink 71 heated to about 430° C. (Aufoil now shown is affixed beforehand to the chip bonding portion of theheat sink to facilitate the formation of AuSi eutectic), allowing AuSieutectic 111 to be developed for fusion-bonding.

In this case, scrubbing is necessary for preventing the generation ofvoids in AuSi, and the larger the size of chip, the greater the degreeof scrubbing is needed. Since it is necessary for the collet 100 toembrace the FET chip, the size of the collet is larger than that of theFET chip 80.

It is necessary that a space margin be ensured an amount sufficient toprevent abutment of a collet end against the ceramic base 74 whenscrubbing width is added to the collet size. Further, it is necessarythat a positional variation margin of the frame 72, a ceramic sizevariation margin, and a collet position accuracy margin be addedthereto. The total is about 1 mm.

FIG. 35 is a sectional view as seen from a package side face forexplaining the space margin of a bonding posts 76 a necessary for wirebonding. In wire bonding, with use of a wire bonding tool 113, wire 112is first-bonded (1st bonding) to an Al electrode (not shown) of the FETchip 80, then is second-bonded (2nd bonding) to an Al electrode (notshown) of the capacitor chip 81 on the drain lead 77 side in theillustrated example. Next, with an electrode (not shown) of thecapacitor chip 81 as the 1st bonding side, the wire is moved onto thebonding post 76 a of the ceramic base 74 to which the drain lead 77 isfixed, and the wire 112 formed of Al is pushed against the bonding post76 a with the bonding tool 113 and is subjected to ultrasoniccompression bonding to effect 2nd bonding. Thereafter, the bonding tool113 is raised while allowing the wire 112 to be clamped by the bondingtool to tear off the wire from the 2nd-bonded portion.

In this case, as shown in FIG. 35, it is necessary to ensure such amargin as prevents abutment of an end portion of the bonding tool 113and the wire 112 against a ceramic sleeve 75. The larger the height ofthe ceramic sleeve 75, or the obtuser the feed angle of the wire 112,the larger is required the length of the bonding post 76 a, thusobstructing the reduction of size.

In a conventional ultrasonic Al wire bonding, the lower the Al wire feedangle, the smaller the variations in compression-bonded shape of Al andthe higher the yield in assembly. For this reason, the lower the Al wirefeed angle, the more desirable. On the other hand, however, it isnecessary that the size of the bonding post 76a be made large. As aresult, there may occur an increase of cost due to an increase inpackage size, or an increase of electrode capacitance may exert aninfluence on characteristics.

Next, with reference to FIGS. 36 to 39, the following description willbe provided about the case where the semiconductor device 70 is attachedto a set radiation plate and also about problems involved therein. FIG.36 is a plan view of the semiconductor device 70 as screwed to a setradiation plate and FIG. 37 is a schematic sectional view thereof.

The semiconductor device 70 is mounted in the following manner. Screws115 are inserted into screwing grooves 73 formed in both ends of theheat sink 71 and the substrate 71 is fixed to a mounting substrate 116with the screws 115. The drain lead 77 and the gate lead 78 areconnected to predetermined wiring portions of the mounting substrate 116through a bonding material (e.g., PbSn solder). In connection withfixing the heat sink 71 as a radiation board to the mounting substrate116, a tightening torque in the fixing work is prescribed forsuppressing an increase in both thermal and electrical resistances.

In actual mounting, for preventing contact imperfection caused by thegeneration of an intermetallic compound between Au plating on thesurfaces of drain and gate leads 77, 78 and PbSn solder, it is necessaryto perform a work for removing Au plating from the leads before themounting. Generally, it is necessary to conduct a preliminary solderingwork of dipping the leads into a solder dipping vessel, allowing the Auplating to be diffused into the solder vessel. Such wasteful labor andtime are required.

As a countermeasure the application of a selective Au plating method isconsidered so as not to plate the lead portions with Au. However, unlikethe selective Au plating for such a material as a lead frame materialcapable of being conveyed continuously, close to a two-dimensionalstructure, masking is difficult for a ceramic package which is of athree-dimensional structure and which is basically conveyed one by one.Thus, the productivity of selective Au plating is low and whole-surfaceAu plating costs lower than selective Au plating. Even if Au plating isthinned for only the leads, there arises the problem of discoloration onheating in chip bonding, so Au plating is applied thick throughout thewhole surface.

Next, problems involved in such screw mounting of the semiconductordevice will be described with reference to FIGS. 37 to 39. FIG. 37 is aschematic sectional view of the studied semiconductor device as screwedto a mounting substrate, FIGS. 38(a) and 38(b) are schematic diagramsshowing the semiconductor device wherein a substrate is warped in acentrally depressed state, as well as a mounted state thereof, and FIGS.39(a) to 39(c) are schematic diagrams showing the semiconductor devicewherein a substrate is warped in a centrally raised state, as well as amounted state thereof.

From the standpoint of heat radiating performance it is preferable thatthe substrate (heat sink) 71 be in such a flat state as shown in FIG.37. However, due to a bimetal effect inducted by thermal stresses amongthe metallic heat sink 71, the ceramic frame 72 fixed to the heat sink71, and the metallic cap 87, there actually occurs a concave warpwherein the substrate center is recessed as in FIGS. 38(a) and 38(b) ora convex warp wherein the substrate center is raised as in FIGS. 39(a)to 39(c). The heat sink 71 and the frame 72 are bonded together bysilver solder 117. In these figures the ceramic portion is not bent, butactually there also is a case where the ceramic portion is bent.

In the case where the heat sink 71 is concavely warped as in FIG. 38(a),if the heat sink is screwed to the mounting substrate 116, as in FIG.38(b), the heat sink which is in a concavely warped state is correctedto the flat side, so that a tensile stress is imposed on the frame 72formed of a ceramic laminate and there easily occurs a crack C.

On the other hand, in the case of such a convex warp as shown in FIG.39(a), a compressive stress is imposed on the frame 72 formed of aceramic laminate due to a screwing stress as in FIG. 39(b), so thebreakage of ceramic is difficult to occur. However, as shown in FIG.39(c), since a gap G is formed in the heat radiation path to themounting substrate 116, a thermal resistance Rth increases and therearises the problem that both reliability and heat radiating performanceare deteriorated.

As a measure against such warps it is considered to use a sufficientlythick heat sink material so as to make the heat sink difficult to warp,and conduct the selection of a heat sink warp. In this case, however,the material cost increases, with consequent increase in the cost of thesemiconductor device.

It is an object of the present invention to reduce the material cost,reduce the cost of an assembling work including a sealing work, reducethe inspection cost, and thereby reduce the semiconductor devicemanufacturing cost.

It is another object of the present invention to provide a semiconductordevice in which a heat sink is difficult to warp.

The above and other objects and novel features of the present inventionwill become apparent from the following description and the accompanyingdrawings.

Typical inventions disclosed herein will be outlined below.

(1) A semiconductor device comprising:

a heat sink formed of a flat metallic plate having a high thermalconductivity;

a pair of screwing pieces having respective inner end portions connectedwith screws respectively to right and left ends of the heat sink on amain surface side of the heat sink, the screwing pieces further havingouter end portions formed with through spaces for screwing;

one or plural semiconductor chips fixed to the main surface of the heatsink;

a seal member formed of an insulating resin, the seal member covering aback side opposite to the main surface of the heat sink and alsocovering the other portions of the two screwing pieces than the outerend portions;

a first lead having an outer end portion projecting from the seal memberand further having an inner end portion which is positioned within theseal member and whose inner end is arranged near one side face of theheat sink;

a second lead having an outer end portion projecting from the sealmember and further having an inner end portion positioned within theseal member and near another side face of the heat sink; and

a plurality of conductive wires for electrically connecting between theleads and predetermined electrodes of the predetermined semiconductorchip, or between the leads and predetermined electrodes of thepredetermined semiconductor chip and also between predeterminedelectrodes of the predetermined semiconductor chip and predeterminedelectrodes of the predetermined semiconductor chip.

The heat sink is formed using a thick material difficult to deform,while the screwing pieces are formed using a flexible material easy todeform. Each of the screwing pieces is bent in one step at anintermediate portion thereof projecting from the seal member, thenextends so that a back side thereof is positioned on a plane on whichthe back side of the heat sink is positioned, or on a plane close to theplane, the bent portion serving as a buffer portion adapted to deformunder a stress. The inner end portions of the first and second leads areformed with engaging portions for engagement with the resin whichconstitutes the seal member. The first or the second lead is partiallyprovided with a polarity identifying portion. The inner ends of thefirst and second leads are positioned in regions diverted from the heatsink.

A semiconductor chip with a field effect transistor formed thereon andsemiconductor chips with capacitors formed thereon are fixed to the mainsurface of the heat sink in such a manner that the capacitor-formedsemiconductor chips are positioned on both sides in the gate-draindirection of the FET-formed semiconductor chip. A drain electrode formedon an upper surface of the field effect transistor and an upperelectrode of one of the semiconductor chips with capacitors formedthereon are connected with each other through the plural wires, and theupper electrode and the first lead are connected with each other throughthe plural wires. Further, a gate electrode formed on the upper surfaceof the field effect transistor and an upper electrode of the othercapacitor semiconductor chip are connected with each other through theplural wires, and the upper electrode and the second lead are connectedwith each other through the plural wires. In this way there isconstituted a high frequency power amplifier for a base station.

Such a semiconductor device is manufactured through the steps of:

providing a lead frame and a heat sink, the lead frame being constitutedby a metallic plate having one or more product forming portions of apredetermined pattern, the heat sink being constituted by a flatmetallic plate of a high thermal conductivity which is fixed to theproduct forming portions with screws;

fixing one or plural semiconductor chips to a main surface of the heatsink;

superimposing the heat sink on back sides of the product formingportions of the lead frame and fixing the heat sink to the lead framewith screws from a main surface side of the lead frame;

connecting between electrodes of the semiconductor chips and inner endportions of corresponding leads and also between predeterminedelectrodes of predetermined semiconductor chips, using conductive wires;

covering the main surface side of the heat sink and a predeterminedportion of the lead frame with an insulating resin by one-side transfermolding to form an insulating seal member; and

cutting off an unnecessary portion of the lead frame, allowing a backside of the heat sink which is insulative to be exposed to a back sideof the seal member and allowing the leads to be projected from sidefaces of the seal member,

the product forming portion each comprising:

a pair of screwing pieces having respective inner end portions connectedwith screws respectively to right and left ends of the heat sink on themain surface side of the heat sink, the screwing pieces further havingrespective outer end portions projecting to the exterior of the sealmember, the outer end portions having respective through spaces forscrewing which are used at the time of mounting the semiconductordevice;

a first lead having an inner end portion which is positioned within theseal member and whose inner end is arranged near one side face of theheat sink and further having an outer end portion projecting from theseal member; and

a second lead having an inner end portion positioned within the sealmember and near another side face of the heat sink and further having anouter end portion projecting from the seal member,

wherein in the step of cutting off the unnecessary portion of the leadframe, or thereafter, the leads projecting from the side faces of theseal member are bent in one step to form surface-mounted type leads.

In the above semiconductor device manufacturing method, the screwingpieces are cut so that there remain the through spaces for screwing, andthe screwing pieces are bent in one step at intermediate portionsthereof projecting from the seal member, then extend so that back sidesthereof are each positioned on a plane on which the back side of theheat sink is positioned or on a plane close to the plane.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective plan view showing the structure of asemiconductor device according to an embodiment (first embodiment) ofthe present invention;

FIG. 2 is a plan view thereof;

FIG. 3 is a front view thereof;

FIG. 4 is a side view thereof;

FIG. 5 is a schematic sectional view taken along line A—A in FIG. 2;

FIG. 6 is a schematic sectional view taken along line B—B in FIG. 2;

FIG. 7 is an enlarged sectional view showing a part of the semiconductordevice of the first embodiment;

FIG. 8 is a plan view of a lead frame used in manufacturing thesemiconductor device of the first embodiment;

FIG. 9 is a plan view of a substrate used in manufacturing thesemiconductor device of the first embodiment;

FIG. 10 is a side view thereof;

FIGS. 11(a) and 11(b) illustrate the substrate with semiconductor chipsfixed thereto in the manufacture of the semiconductor device of thefirst embodiment;

FIG. 12 is a plan view of the lead frame with the substrate attachedthereto;

FIG. 13 is a plan view of the lead frame after the end of wire bondingin the manufacture of the semiconductor device of the first embodiment;

FIG. 14 is a schematic diagram showing how to perform the wire bonding;

FIG. 15 is a plan view of the lead frame after the end of sealing withresin in the manufacture of the semiconductor device of the firstembodiment;

FIG. 16 is a schematic diagram showing in what state the semiconductordevice of the first embodiment is manufactured as a device equipped withsuspension leads by cutting the lead frame selectively;

FIG. 17 is a schematic diagram showing in what state the semiconductordevice of the first embodiment is manufactured as a device not equippedwith suspension leads by cutting the lead frame selectively;

FIGS. 18(a) and 18(b) are a diagram and a graph, respectively, showinghow a gap formed between a package bottom of the semiconductor deviceand a lower surface of each suspension lead, as well as thermalresistance and package breakdown occurrence rate, are correlated witheach other;

FIG. 19 is a perspective plan view showing the structure of asemiconductor device according to another embodiment (second embodiment)of the present invention;

FIG. 20 is a schematic sectional view thereof;

FIG. 21 is a schematic sectional view in another section of thesemiconductor device of the second embodiment;

FIG. 22 is a plan view of a spacer used in a semiconductor deviceaccording to a modification of the second embodiment;

FIG. 23 is a schematic sectional view showing the structure of asemiconductor device according to another modification of the secondembodiment;

FIG. 24 is a plan view of a semiconductor device according to a furtherembodiment (third embodiment) of the present invention;

FIG. 25 is a front view of thereof;

FIG. 26 is a plan view of a semiconductor device according to a stillfurther embodiment (fourth embodiment) of the present invention;

FIG. 27 is a side view thereof;

FIG. 28 is a plan view of the studied semiconductor device which thepresent inventor had studied prior to the present invention;

FIG. 29 is a side view thereof;

FIG. 30 is a plan view thereof with a cap removed;

FIG. 31 is a schematic sectional view thereof;

FIG. 32 is an equivalent circuit diagram thereof;

FIG. 33 is a schematic sectional view of an FET chip used in the studiedsemiconductor device;

FIG. 34 is a schematic diagram showing a state of chip bonding in themanufacture of the studied semiconductor device;

FIG. 35 is a schematic diagram showing a state of wire bonding in themanufacture of the studied semiconductor device;

FIG. 36 is a plan view of the studied semiconductor device as screwed toa set radiation board;

FIG. 37 is a schematic sectional view thereof;

FIGS. 38(a) and 38(b) are schematic diagrams showing a mounted state ofthe studied semiconductor device in which a substrate is warped in acentrally depressed state; and

FIGS. 39(a) to 39(c) are schematic diagrams showing a mounted state ofthe studied semiconductor device in which a substrate is warped in acentrally raised state.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described in detailhereinunder with reference to the accompanying drawings. In all of thedrawings for illustrating the embodiments, portions having the samefunctions are identified by like reference numerals, and repeatedexplanations thereof will be omitted.

First Embodiment

FIGS. 1 to 18 are concerned with a semiconductor device according to anembodiment (first embodiment) of the present invention, of which FIGS. 1to 7 are concerned with the structure of the semiconductor device andFIGS. 8 to 17 are concerned with the manufacture of the semiconductordevice. In this first embodiment, a description will be given belowabout an example in which the present invention is applied to a highfrequency power amplifier (a high frequency power amplifier module) fora base station, as a semiconductor device.

The semiconductor device of this first embodiment, i.e., a highfrequency power amplifier module for a base station, indicated at 1, hassuch a structure as shown in FIGS. 1 to 7, of which FIG. 1 is aperspective plan view, FIG. 2 is a plan view, FIG. 3 is a front view,FIG. 4 is a side view, FIGS. 5 and 6 are schematic sectional views takenalong lines A—A and B—B in FIG. 2, and FIG. 7 is a partially enlargedsectional view.

In appearance, as shown in FIGS. 2 to 4, the semiconductor device 1 ofthis embodiment comprises a flat, rectangular, seal member (package) 2formed of an insulating resin, a heat sink (substrate) 3 whose lowersurface is exposed to a bottom of the seal member 2, a pair of screwingpieces (suspension leads) 4 projecting respectively from both ends(short sides) in the longitudinal direction of the seal member 2, afirst broad lead 5 and a second broad lead 6 projecting respectivelyfrom long sides of the seal member 2. The second lead 6 serves as a gate(G) lead. The lower surface of the heat sink 3 exposed to the bottom ofthe seal member 2 serves as both a heat radiating surface and a sourceelectrode.

The seal member 2 is formed, for example, using an insulating epoxyresin by transfer molding. The seal member 2 has a structure whichcovers the four side faces of the heat sink 3 and in which a wateradmission path passing the interface between the heat sink 3 and theresin is made long to ensure a high reliability. Recesses 8 are formedcentrally of short sides of the seal member 2 and toward the center ofthe seal member. The recesses 8 are draw-out traces of an upper mold forclamping the heat sink 3 and the screwing pieces 4 with upper and lowermolds of a molding die and causing the heat sink 3 to come into closecontact with the lower mold positively in transfer molding to preventthe occurrence of burrs caused by flowing-out of resin.

The heat sink 3 and the screwing pieces 4 are formed of metal, andwithin the seal member 2, an inner end portion of each screwing piece 4is superimposed on an upper surface of the heat sink 3 and is connectedthereto with two screws 7 (see FIG. 1), so that the heat sink 3 and thescrewing pieces 4 become equal in potential and can be used as source(S) terminals. Semiconductor chips are fixed to the heat sink 3 and theheat sink plays the role of dissipating the heat generated in thesemiconductor chips, therefore, the heat sink 3 is formed of a flatmetallic plate (e.g., CuMo plate) superior in thermal conductivity.Besides, the heat sink 3 is as thick as 2 mm for example to make itsdeformation difficult. On the other hand, the screwing pieces 4 are asthin as 0.15 mm for example and are flexible so as to serve assuspension leads.

Both screwing pieces 4 have screwing grooves 9 formed in respectiveouter end portions. With use of the paired screwing grooves 9 thesemiconductor device 1 is fixed with screws to a mounting substrate suchas a mother board. The inner end portions of the screwing pieces 4 aresuperimposed and fixed onto the main surface (upper surface) of the heatsink 3, and the outer end portions of the screwing pieces 4 are mountedin contact with the mounting substrate. Therefore, as shown in FIG. 5,the screwing pieces 4 are bent in one step at their portions projectingfrom the seal member 2. The bent portions, indicated at 10, are easilydeformable portions against a stress, i.e., suspension portions.

As shown in FIG. 7, a plating film 11 is formed on the whole surface ofthe heat sink 3. The plating film 11 comprises Ni plating film asundercoat and Au plating film formed on the Ni plating film. The Niplating film is superior in its adhesion to CuMo which constitutes theheat sink 3. The Au plating film is formed as thick as about 3 μm sothat it can withstand heat of a high temperature which is generated atthe time of fixing semiconductor chips to the heat sink 3 by AuSieutectic.

Each screwing piece 4 also has a plating film 12 formed on its surface.The screwing piece 4 is formed of an FeNi-base metal such as Kovar or 42alloy. The plating film 12 comprises Ni plating film as undercoat and Auplating film formed on the Ni plating film. The Ni plating film issuperior in its adhesion to the FeNi-base metal. The Au plating film isformed for improving the bondability of aluminum (Al) wire, but itsthickness may be as small as 0.2 μm or so.

As shown in FIGS. 1 and 5 to 7, three semiconductor chips 15, 16, and 17are fixed to the main surface portion of the heat sink 3 located betweenthe pair of screwing pieces 4. The semiconductor chips 15, 16, and 17are elongated in shape and with electrodes extending along their longsides. All the semiconductor chips 15, 16, and 17 extend along the longsides of the heat sink 3. The long sides of the seal member 2 and thoseof the heat sink 3 extend in the same direction and are opposed to eachother.

A field effect transistor (MOSFET) is incorporated in the semiconductorchip 15. As shown in FIG. 7, out of drain, gate, and source electrodesof MOSFET, a drain electrode 20 and a gate electrode 21 are formed on anupper surface of the semiconductor chip 15, while a source electrode 22is formed on a lower surface of the chip 15. The drain and gateelectrodes are elongated and extend along long sides of the chip in anopposed relation to each other. The source electrode 22 is fixed to theheat sink 3 through an AuSi eutectic layer 23. The AuSi eutectic layer23 is formed by affixing gold foil to a predetermined portion of theheat sink 3, then pushing the semiconductor chip against the gold foil,followed by heating scrubbing.

The semiconductor chips 16 and 17 are fixed onto the main surface of theheat sink 3 on both sides of the semiconductor chip 15 spacedly apredetermined distance from the chip 15. The semiconductor chips 16 and17 have a structure in which capacitors are incorporated. As shown inFIGS. 6 and 7, one upper electrodes 25 a and 25 b are formed on uppersurfaces of the semiconductor chips 16 and 17 respectively, while otherlower electrodes 26 a and 26 b are formed on lower surfaces of thesemiconductor chips 16 and 17 respectively. The lower electrodes 26 aand 26 b of the semiconductor chips 16 and 17 are fixed to the heat sink3 through AuSi eutectic layers 27 a and 27 b respectively. As is thecase with the semiconductor chip 15, the AuSi eutectic layers 27 a and27 b are formed by affixing gold foil to predetermined portions of theheat sink 3 and thereafter pushing the semiconductor chips against theheat sink 3, followed by scrubbing under heating. As a result, thesemiconductor chips 16 and 17 are fixed to the heat sink 3.

The drain lead 5 and the gate lead 6 have a broad structure so as tostand face to face with the elongated semiconductor chips 15, 16, and17. The drain electrode 20 of the semiconductor chip 15 and the upperelectrode 25 a of the semiconductor chip 16 located close to the drainlead 5 are electrically connected with each other through a conductivewire 31. Further, the upper electrode 25 a and the drain lead 5 areelectrically connected with each other through a conductive wire 32.Likewise, the gate electrode 21 of the semiconductor chip 15 and theupper electrode 25 b (see FIG. 1) of the semiconductor chip 16 locatedclose to the gate lead 6 are electrically connected with each otherthrough a conductive wire 33. Further, the upper electrode 25 b and thegate lead 6 are electrically connected with each other through aconductive wire 34.

As to the wires for connection between electrodes or between electrodesand leads, as shown in FIG. 1, plural wires of connections can beeffected because the wire connections are parallel to one another andextend long. By this plural-wire connection it is possible to ensuredesired current capacity and high frequency characteristics (e.g., drainefficiency and gain). As the wires there are used aluminum wires. Thealuminum wires are each in the form of a wire loop so as to afford anappropriate inductance for a desired frequency characteristic and permitthe formation of a desired internal circuit, whereby there is formed ahigh frequency power amplifier module constituted by the equivalentcircuit shown in FIG. 32.

As shown in FIG. 1, each lead portion positioned within the seal member2 is provided with engaging portions 35 on both sides of the lead forengagement with the resin which constitutes the seal member. This is forpreventing easy release of the leads 5 and 6 from the seal member 2.Further, a corner portion of an outer end of the drain lead 5 is cut offobliquely to form a polarity identifying portion 36 so that the polarityof the electrode can be identified visually. The shape and structure ofthe polarity identifying portion 36 may be other shape and structure.

The leads 5 and 6 and the screwing pieces (suspension leads) 4 areformed by cutting a single lead frame in the manufacture of thesemiconductor device. Therefore, the composition, thickness and surfacetreatment are common to them.

Next, with reference to FIGS. 8 to 17, the following description isprovided about the method of manufacturing the semiconductor deviceaccording to the first embodiment. In this first embodiment there areused a lead frame 40 shown in FIG. 8 and a heat sink (substrate) 3 shownin FIGS. 9 and 10.

The lead frame 40 is formed into a predetermined pattern by etching ametallic plate about 0.15 mm in thickness or by punching the plate witha precision press. The lead frame 40 used in this embodiment is of amulti-arrangement structure in which plural product forming portions 41each for the formation of one product are arranged in a row. In thisembodiment the number of the product forming portions 41 is five. Themetallic plate is formed using an FE—Ni-base alloy such as Kovar or 42alloy to match the heat sink 3 in expansion coefficient. On one side ofthe lead frame 40 are formed guide holes 43 to be used for conveyance orpositioning of the lead frame. The guide holes 43 are positioned inopposition to the product forming portions 41 respectively.

As shown on the right end side of FIG. 8, each product forming portion41 has a quadrangular frame 45, a pair of cantilevered screwing pieces 4a projecting inwards from central portions of a pair of opposed sides ofthe frame 45, and cantilevered leads 5 a and 6 a projecting inwards fromcentral portions of the remaining pair of opposed sides. At a baseportion of the frame 45 one side edge of the lead 5 a is thinnedobliquely to form a polarity identifying portion 36. In a subsequentstep of cutting the lead 5 a the polarity identifying portion 36 is cuthalfway thereof to form a polarity identifying portion 36 at a tipcorner portion of the lead 5.

Ni plating film is formed on the whole surface of the lead frame 40 andAu plating film is formed at least on surfaces of the screwing pieces 4a and the leads 5 a, 6 a. Ni plating film is high in its adhesion toboth Fe—Ni alloy and Au plating. In this first embodiment it sufficesfor the Au plating film to be as thin as 0.2 μm for example because itis only the connection of Al wires to the leads 5 a and 6 a that isrequired. The material cost is reduced by thinning the Au plating film.

All of the screwing pieces 4 a and the leads 5 a, 6 a are eventually cutinto screwing pieces 4 and leads 5, 6 at positions near base positionsof the frame 45. Therefore, in each screwing piece 4 a is formed a hole9 a by a pattern which partially overlaps a screwing groove 9 so thatthe groove 9 is formed upon formation of the screwing piece 4. Further,recessed engaging portions 35 are formed on both sides of an inner endportion of each of the leads 5 a and 6 a.

A square space is present centrally of each product forming portion 41.The heat sink (substrate) 3 comes to be positioned in the space. Screwsare used in this embodiment for fixing the heat sink 3. Therefore, guideholes 46 for insertion therein of the screws are formed in both sideedge portions on an inner-end side of each screwing piece 4. FIGS. 9 and10 show a rectangular heat sink 3, with tapped holes 47 being formed inthe four corners of the heat sink correspondingly to the guide holes 46.Means for fixing the heat sink 3 is not limited to the use of screws.There may be adopted welding, soldering, or riveting.

Semiconductor chips are fixed to the heat sink 3. The semiconductorchips are each formed on a fragile silicon board, so for the preventionof damage to each semiconductor chip, it is required that the heat sink3 be high in rigidity. It is also required for the heat sink 3 to behigh in thermal conductivity because it also plays the role ofdissipating the heat generated in the semiconductor chips to theexterior promptly. For this reason, the heat sink 3 is formed using sucha metallic material as is highly compatible in thermal expansioncoefficient with the semiconductor chips formed of silicon and superiorin heat radiating property. For example, the heat sink 3 is formed of aflat CuMo plate about 2 mm thick. Ni plating film high in adhesion toCuMo is formed on the surface of the heat sink 3 and Au plating film isformed on the Ni plating film. The Au plating film is as thick as 3 μmor so. This is for making the semiconductor chips resistive to ahigh-temperature heat at the time of chip bonding for fixing the chipsto the heat sink 3 by AuSi eutectic.

At the beginning of the assembling work in this first embodiment, asshown in FIGS. 11(a) and 11(b), three semiconductor chips 15, 16, and 17are fixed to the main surface of the heat sink 3 by AuSi eutectic layer.This fixing operation can be done easily by affixing gold foil topredetermined portions of the heat sink, then pushing the semiconductorchips against the gold foil, followed by scrubbing under heating (430°C. for example). In this way the semiconductor chip 15 with MOSFETformed thereon is fixed centrally and the semiconductor chips 16 and 17with capacitors formed thereon are fixed on both sides of the chip 15(see FIGS. 6 and 7).

This chip bonding work is performed by means of a chip bonding apparatus(not shown). Handling of each semiconductor chip is conducted using atool called collet. The heat sink 3 is a flat plate having a flat mainsurface. Thus, unlike the heat sink 71 shown in FIG. 34, any obstacle,including the frame 72, is not present on the main surface, so there isno fear of the collet striking against any obstacle even when it isscrubbed, thus permitting bonding for any portion of the main surface ofthe heat sink 3. This leads to reduction of the bonding margin and hencereduction in size of the heat sink 3.

Next, the heat sink 3 is fixed to the lead frame 40, as shown in FIG.12. More specifically, the main surface of the heat sink 3 is positionedand superimposed on the back side of the lead frame 40, then screws 7are inserted into the guide holes 46 formed in the lead frame 40 andtheir tips are threaded into the tapped holes 47 of the heat sink 3,whereby the heat sink is fixed to the lead frame 40. At this stage, thediscrete members are assembled into a continuous frame shape, thuspermitting easy handling and permitting subsequent works to be done inhigh efficiency and accurately.

Next, wire bonding is conducted as shown in FIGS. 13 and 14. The wirebonding is carried out by means of an ultrasonic wire bonder using Alwire. More specifically, as shown in FIG. 14, the lead frame 40 with theheat sink 3 secured thereto is put on a stage 50 of the ultrasonic wirebonder and then a wire 52 held by a bonding tool 51 is connected byscrubbing induced by ultrasonic oscillation of the bonding tool 51. Forexample, the drain electrode 20 of the semiconductor chip 15 on the heatsink 3 and the upper electrode 25 a on the semiconductor chip 16 areconnected with each other through wires 31, while the upper electrode 25a and the lead 5 a are connected with each other through wires 32.

Connection of the wires 31 and 32 is performed as indicated with arrowsa to j in FIG. 14. To be more specific, the bonding tool 51 moves downlike arrow a while holding the wire 52 with use of a clamper, pushes thetip of the wire 52 against the drain electrode 20, then oscillates forconnection (first bonding), thereafter moves upward, horizontally andthen downward like arrows b, c, d, to connect an intermediate portion ofthe wire 52 to the upper electrode 25a on the semiconductor chip 16(second bonding). During such ascent, horizontal movement, and descentof the bonding tool 51, the damper opens and the wire 52 is drawn outsuccessively from the tip of the bonding tool 51, so that the wire 31 isconnected while describing a desired loop.

After the second bonding, the bonding tool 51 rises like arrow e, butthe damper holds the wire 52 during this upward movement. As a result,the wire 52 is pulled and breaks at a position near the connection(second bonding portion) with the upper electrode 25 a.

Next, the bonding tool 51 moves like arrows f to j to effect connectionof the wire 32 in which the upper electrode 25 a on the semiconductorchip 16 is the first bonding portion and the tip of the lead 5 a is thesecond bonding portion. As a result, the drain electrode 20 on thesemiconductor chip 15 is connected to the lead 5 a electrically.

Next, the stage 50 is turned 180°, then by the same wire bonding methodas that described previously the gate electrode 21 of the semiconductorchip 15 and the upper electrode 25 b of the semiconductor chip 17 areconnected with each other through wires 33, and the upper electrode 25 band the lead 6 a are connected with each other through a wire 34, tocomplete wire bonding. The wires 31 to 34 are in such connected statesas shown in FIG. 1.

Since the Al wire bonding work can be done at room temperature, thedeterioration of the surface plating of the lead frame 40 is difficultto occur. Therefore, even if the Au plating film formed on the surfacesof leads 5 a and 6 a of the lead frame 40 is as thin as 0.2 μ or so, theproblem of deteriorated bondability does not occur. If the Au platingfilm is as thin as 0.2 μm or so, it is possible to enhance thereliability because an intermetallic compound of AlAu cannot grow. Therealso accrues an actual advantage that the work for removing Au from thelead surfaces is not needed at the time of mounting performed by a user.

The wire bonding work can be done smoothly because an obstacle such as aframe which interferes with the bonding tool 51 in wire bonding andrestricts the movement of the bonding tool is not present on the mainsurface of the heat sink 3.

Next, one-side molding is performed by transfer molding. FIG. 15 is aplan view of the lead frame 40 in which the heat sink 3 and the wires 31to 34 are covered with the seal member 2. As shown in the same figure, alead 5 a projects from one long side of the seal member 2 which is flatand quadrangular in shape, a lead 6 a projects from the other long side,and screwing pieces 4 a project from short sides respectively. As amatter of course, holes 9 a are positioned outside the seal member 2.

In central edge portions of the short sides of the seal member 2 areformed recesses 8 which are depressed toward the center of the sealmember. The recesses 8 are draw-out traces of an upper mold of a moldingdie which pushes the heat sink 3 and the screwing pieces 4 asuperimposed on the heat sink against a lower mold of the die intransfer molding. With the upper mold which leaves such draw-out traces,the screwing pieces 4 a can be brought into close contact with the heatsink 3 and the back side of the heat sink can be put in close contactwith the lower mold, whereby it is possible to prevent the leakage ofresin.

Next, an unnecessary lead frame portion is removed to manufacture thesemiconductor device. In the upper-stage portions of FIGS. 16 and 17, aquadrangular shape which surrounds each seal member 2 represents acutting line 55 in a cutting die, while the lower-stage portionsrepresent the semiconductor device 1 manufactured by cutting along thecutting line 55.

Cutting and removal of an unnecessary lead frame portion are carried outby cutting the pair of screwing pieces 4 and leads 5 a, 6 a projectingfrom the seal member 2 rectilinearly in their width directions.

As shown in FIG. 16, by cutting the leads 5 a and 6 a at positionsspaced a predetermined distance from edges of the seal member 2 therecan be obtained leads 5 and 6 as external electrode terminals. Incutting the leads 5 a and 6 a, the lead 5 a is cut intermediate thepolarity identifying portion 36 which becomes an oblique portion. As aresult, an obliquely cut-out polar identifying portion 36 is formed atone corner of the outer end of the lead 5. Further, the screwing pieces4 a are cut so as to cross the holes 9 a, whereby screwing pieces(suspension leads) 4 having screwing grooves 9 at respective outer endscan be formed, as shown in FIG. 2.

In this case, the cutting positions of the screwing pieces 4 a may beset close to sides of the seal member 2, as shown in FIG. 17, whereby itis possible to prevent screwing grooves from being formed in the outerends of the screwing pieces 4. In the semiconductor device 1 thusfabricated, the short screwing pieces 4 projecting from the seal member2 are not used in mounting.

In any case, the leads 5 and 6 serve as a drain lead 5 and a gate lead6, respectively. Further, the heat sink serves as a source electrode.The screwing pieces (suspension leads) 4 connected to the heat sink 3can be used also as source electrodes (source leads).

Next, there are performed characteristic selection (electricalselection) and a mark forming work. Further, by bending in one step thepair of screwing grooves 4 projecting from side faces of the seal member2 to form such gull wing-like leads as shown in FIG. 3, it is possibleto manufacture such a semiconductor device 1 as shown in FIG. 3 in aplural number.

The back sides of the screwing pieces 4 are positioned on the same planeas the back side of the heat sink 3 or on a plane close thereto. Thisbending work may be done simultaneously with the cutting of the screwingpieces 4.

FIGS. 18(a) and 18(b) are a diagram and a graph, respectively, showinghow a gap (t) formed between a package bottom (the bottom of the heatsink 3) in the semiconductor device 1 manufactured by the method of thefirst embodiment and a lower surface of each screwing piece (suspensionlead) 4 is correlated with a thermal resistance and a package breakdownoccurrence rate. More specifically, FIG. 18(a) is a schematic diagram ofthe semiconductor device 1, showing the gap (t), and FIG. 18(b) is agraph showing a correlation between the gap (t) and a thermal resistanceas well as a package breakdown occurrence rate.

As is seen from the above graph, in the semiconductor device of thestructure according to the present invention (the first embodiment) andin case of the gap (t) being minus (−), the larger the minus, the widerthe gap from the mounting substrate at the time of mounting, with theresult that the thermal resistance through the heat sink 3 increasesrapidly. In case of the gap (t) being plus (+), the thermal resistancebecomes constant because the heat sink 3 comes into contact with themounting substrate.

In the semiconductor device of this first embodiment, by making the gap(t) plus, that is, by setting the lower surface of each suspension leadat a position higher than the back side (lower surface) of the heat sink3, i.e., at a retracted position from the back side of the heat sink 3,it is possible to diminish the thermal resistance and also possible tomake the package breakdown occurrence rate smaller than in the case ofthe studied semiconductor device shown in FIG. 28 and subsequentfigures. In the graph of FIG. 18(b), the measurement result on thesemiconductor device of the structure according to the present inventionhas been obtained in a screwed state of the suspension leads 4 to themounting substrate, while the measurement result on the structure of thestudied semiconductor device has been obtained in a fixed state of theheat sink to the mounting substrate directly with screws.

The following effects are obtained by the semiconductor device andmanufacturing method for the same according to the first embodiment.

(1) According to the structure of the semiconductor device, the heatsink for heat radiation is thick, while the screwing pieces to be usedin mounting the semiconductor device are thin, whereby a stress inducedin mounting is absorbed by deformation of the stepped portions of thethin screwing pieces. Therefore, the heat sink no longer undergoes sucha deformation as warping and hence it is possible to prevent damage ofthe semiconductor chips fixed to the heat sink.

(2) Since the set screw space so far provided in the heat sink 3 can beomitted, it is possible to decrease the amount of the expensive CuMomaterial used and hence possible to reduce the cost of the semiconductordevice 1.

(3) Since the screwing pieces (suspension leads) 4 are thinner than theheat sink 3 and permit the selection of a material low in rigidity andsuperior in formability, a stress induced at the time of screwing in themounting work can be absorbed by the bent portions 10 of the suspensionleads 4. As a result, the heat sink machining accuracy can be mitigatedand it is possible to reduce the package cost.

(4) Since it is only the heat sink portion that is required to have heatresistance in AuSi eutectic chip bonding, it suffices for only the heatsink 3 to be thick plated with Au. That is, it is possible to decreasethe amount of the expensive Au used an hence possible to reduce the costof the semiconductor device 1.

(5) In the resin molded package, in comparison with a ceramic package,both sealing material cost and machining cost are low; besides, hermeticseal inspection and dust particle inspection are not needed, thus makingcontribution to the reduction of cost and making it possible to reducethe cost of the semiconductor device 1.

(6) Since the plating of the leads 5 and 6 is an extremely thin Auplating, an Au plating removing work is not required on the user side,whereby the reduction of mounting cost can be attained. If the surfacesof the leads 5 and 6 are plated with any other plating material than Au,e.g., solder, it becomes unnecessary for the user to perform the Auplating removing work, whereby it is possible to attain the reduction ofthe mounting cost.

(7) By adoption of the resin molded package it becomes easy to attainthe reduction in size and high integration of the semiconductor devicein comparison with the ceramic package. More particularly, in thesemiconductor device having a ceramic package, the ceramic package iscompelled to become large in size in order to avoid contact of the chipholding collet with inner wall surfaces of the package at the time ofchip bonding. On the other hand, in the case of a resin molded package,the size thereof can be reduced because chip bonding and wire bondingare performed in the absence of a package and thereafter the resinmolded package is formed.

(8) Since the Al wire bonding work can be done at room temperature, theplating on the surface of the lead frame 40 is difficult to occur.Therefore, even if the Au plating formed on the surfaces of the leads 5a and 6 a of the lead frame 40 is as thin as 0.2 μm or so, there doesnot arise any problem related to bondability. With a thin Au platingfilm of 0.2 μm or so, an intermetallic compound of AlAu cannot grow andhence it is possible to enhance the reliability. There also accrues anactual advantage that the Au removing work from the lead surfaces is notneeded at the time of mounting performed on the user side.

(9) The lead frame and the heat sink are used in this first embodiment,but the heat sink is screwed to each of the product forming portions 41,so in the subsequent assembling work there can be utilized an assemblingtechnique with the thus-assembled lead frame, whereby the assemblingwork can be done efficiently and consequently it is possible to reducethe semiconductor device manufacturing cost.

Second Embodiment

FIGS. 19 to 21 illustrate a semiconductor device according to anotherembodiment (second embodiment) of the present invention, of which FIG.19 is a perspective plan view showing the structure of the semiconductordevice, FIG. 20 is a schematic sectional view of the semiconductordevice, and FIG. 21 is a schematic sectional view in another section ofthe semiconductor device.

According to the semiconductor device of this second embodiment, in thestructure of the first embodiment there is adopted a double moldedstructure in which, as shown in FIGS. 19 to 21, the semiconductor chips15, 16, 17 and the wires 31 to 34 are covered with undercoat resin 56,and the undercoat resin 56, etc. are sealed with the seal member 2.

The heat resistance of transfer molding resins presently in practicaluse is 150° C. or so, thus giving rise to the problem that it isimpossible to cope with the junction temperature of Tj≧175° C. requiredof a power FET for a base station. Besides, there is a fear that a highfrequency loss of a transfer molding resin may become unignorableagainst the market trend to an increase of operating frequency.

In view of this point, according to the structure of this secondembodiment, the semiconductor chips 15, 16, 17, and the wires 31 to 34are covered with a potting type undercoat resin superior in heatresistance and with little high frequency loss. By such a double moldedstructure using the under coat resin the chip surfaces which become thehighest in temperature in the semiconductor device and the bonding wireportion as a high frequency current path can be covered with a pottingresin having high heat resistance and low loss characteristics can becovered with the potting resin. Thus, the structure in question iseffective for a semiconductor device for a radio base station with ajunction temperature as high as 150 to 200° C. Therefore, this structureis effectively applicable to a semiconductor device of a high frequencyoutput of 50W or more.

For the prevention of wire breaking during the application of theundercoat resin it is preferable that the whole surfaces of the chipsand the wires be covered with the undercoat resin. But in the case ofthe structure of the previous first embodiment, the undercoat resinflows down from the gap between the leads and the heat sink, making itimpossible to cover the whole of the wires.

In this second embodiment, in view of the point just mentioned above,the inner ends of the leads 5 and 6 are positioned on the heat sink 3 toprevent flowing and loss of the undercoat resin 56, as shown in FIG. 19.In this case, for electrical insulation between the leads 5, 6 and theheat sink 3, as shown in FIG. 20, spacers 57 each constituted by adoughnut-like insulator are sandwiched in between the screwing pieces 4a and the heat sink 3 to ensure insulation between the heat sink and thescrewing pieces 4 a. Thus interposing the spacers 57 between the heatsink 3 and the screwing pieces 4 a means that in the assembled state thescrewing pieces 4 a and the leads 5, 6 (leads 5 a, 6 a) are spaced aparta distance corresponding to the thickness of each spacer 57.

Consequently, at the time of application of the undercoat resin, theresin which has flowed down between the inner end sides of the leads 5a, 6 a and the wires 31 to 34 gets on the heat sink 3 and no longerflows away, but covers the wires 31 to 34, further covers thesemiconductor chips 15, 16, 17 and the leads 5 a, 6 a.

According to such a structure wherein the spacers 57 are used to ensuregaps between the heat sink and the lead frame (leads 5 a, 6 a, and thescrewing pieces 4 a), at the time of clamping with a molding die intransfer molding, the screwing pieces 4 a (4) are deformed when the heatsink 3 is pushed against the lower mold of the die, and thus there is afear that the clamping operation may not be carried out in asatisfactory manner. In this second embodiment, therefore, holes (uppermold through holes) 58 for passage therethrough of the upper mold areformed in the screwing pieces 4 a to prevent deformation of the screwingpieces 4 a. That is, the holes 58 are heat sink pressing type windows.

In this second embodiment, the undercoat resin material and the resinmaterial which forms the seal member can be selected in accordance withseparate specifications, it is possible to easily improve thecharacteristics and reliability of the semiconductor device 1.

FIG. 22 is a plan view of a spacer used in a semiconductor deviceaccording to a modification of the second embodiment. In the illustratedexample, spacers 57 formed of an insulator and having an external shapealmost equal to that of the heat sink 3 are interposed between the heatsink 3 and the leads 5 a, 6 a (5, 6) as well as the screwing pieces 4 a(4). The spacers 57 a are formed with tapped holes 59 for insertiontherein of screws 7.

Since the insulating spacers 57 a have such a structure as illustratedin the figure, they are disposed also under the leads 5 a and 6 a (5,6), so that the insulation between the leads 5, 6 and the heat sink 3 isensured and the reliability of the semiconductor device 1 is improved.

According to this structure, moreover, vertical movements (fluttering)of the leads 5 and 6 are diminished, ultrasonic oscillation in wirebonding can surely be applied to the wire, thus permitting a highlyaccurate wire bonding. As a result, it is possible to provide asemiconductor device 1 with high wire connectability.

The leads of a semiconductor device used in a high frequency amplifiercircuit constitute a part of a high frequency circuit, so an appropriatecapacitance is required as the case may be. When such spacers 27 a asshown in FIG. 22 are interposed between the leads 5, 6 and the heat sink3, they function as dielectrics and therefore a required capacitance canbe obtained by combining lead area, lead-heat sink spacing, andlead-heat sink dielectric constant with one another, also permitting theimprovement in efficiency of the high frequency amplification circuit.

FIG. 23 is a schematic sectional view showing the structure of asemiconductor device according to another modification of the secondembodiment. In this modification there is proposed another structure forensuring a gap between the lead frame and the heat sink. Morespecifically, as shown in the same figure, the portions of the screwingpieces 4 a (4) where guide holes 46 are to be formed are subjected toembossing and are thereby formed lower than the portions adjacentthereto, and the lower portions, indicated at 60, are fixed to the heatsink 3 with screws 7, whereby the screwing pieces 4 a and the leads 5 a,6 a (5, 6) are made higher by a predetermined spacing from the mainsurface of the heat sink 3 to form gaps.

Third Embodiment

FIG. 24 is a plan view of a semiconductor device according to a furtherembodiment (third embodiment) of the present invention and FIG. 25 is afront view thereof. This third embodiment shows an example in which thedrain and gate leads 5, 6 of the structures described in the firstembodiment are each arranged side by side in a plural number. In thisthird embodiment, two sets of drain and gate leads 5, 6 are arrangedside by side. With such a structure, it is possible to increase theoutput.

Fourth Embodiment

FIG. 26 is a plan view of a semiconductor device according to a stillfurther embodiment (fourth embodiment) of the present invention and FIG.27 is a side view thereof. In this fourth embodiment the presentinvention is applied to a ceramic package.

According to the semiconductor device, indicated at 1 a, of this fourthembodiment, in the studied semiconductor device, a metallic cap 87 isextended long on both end sides of the frame 72 constituted by a ceramiclaminate, and extended portions 87 d are bent in one step, then extendso that lower surfaces thereof are positioned on a plane coincident withor close to the back side of the heat sink 71. In outer ends of theextended portions 87 d are formed screwing grooves 73.

The cap 87 is bonded through AuSn alloy 89 to the plating film 87 aformed on an upper surface of the frame 72. The cap 87 is formed so asto become equal in potential as the source electrode. Thus, the extendedportions 87 d are employable also as source electrodes.

Although the present invention has been described above on the basis ofembodiments thereof, it goes without saying that the invention is notlimited to those embodiments and that various changes may be made withinthe scope not departing from the gist thereof. Although in the aboveembodiments screws are used to mount the respective semiconductordevices, the invention is also applicable to semiconductor devices whichare mounted by the reflow of solder.

The following is a brief description of effects obtained by typicalinventions disclosed herein.

(1) It is possible to reduce the material cost, reduce the cost ofassembling, including sealing, and further reduce the inspection cost,and hence possible to reduce the semiconductor device manufacturingcost.

(2) It is possible to provide a semiconductor device wherein the warp ofa heat sink is difficult to occur, and hence possible to improve thereliability of mounting.

What is claimed is:
 1. A method of manufacturing a semiconductor device,comprising the steps of: providing a lead frame and a heat sink, thelead frame being constituted by a metallic plate having one or moreproduct forming portions of a predetermined pattern, the heat sink beingconstituted by a flat metallic plate of a high thermal conductivitywhich is fixed to the product forming portions with screws; fixing oneor plural semiconductor chips to a main surface of the heat sink;superimposing the heat sink on back sides of the product formingportions of the lead frame and fixing the heat sink to the lead framewith screws from a main surface side of the lead frame; connectingbetween electrodes of the semiconductor chips and inner ends ofcorresponding leads and also between predetermined electrodes ofpredetermined semiconductor chips, using conductive wires; covering themain surface side of the heat sink and a predetermined portion of thelead frame with an insulating resin by one-side transfer molding to forman insulating seal member; and cutting off an unnecessary portion of thelead frame, allowing a back side of the heat sink which is insulative tobe exposed to a back side of the seal member and allowing the leads tobe projected from side faces of the seal member, the product formingportions each comprising: a pair of screwing pieces having respectiveinner end portions connected with screws respectively to right and leftends of the heat sink on the main surface side of the heat sink, thescrewing pieces further having outer end portions projecting to theexterior of the seal member, the outer end portions having respectivethrough spaces for screwing which are used at the time of mounting thesemiconductor device; a first lead having an inner end portion which ispositioned within the seal member and whose inner end is arranged nearone side face of the heat sink and further having an outer end portionprojecting from the seal member; and a second lead having an inner endportion positioned within the seal member and near another side face ofthe heat sink and further having an outer end portion projecting fromthe seal member, wherein in the step of cutting off the unnecessaryportion of the lead frame, or thereafter, the leads projecting from theside faces of the seal member are bent in one step to formsurface-mounted type leads.
 2. A method according to claim 1, whereinthe screwing pieces are cut so that there remain the through spaces forscrewing, and the screwing pieces are bent in one step at intermediateportions thereof extending from the seal member, then extend so thatback sides thereof are each positioned on a plane on which the back sideof the heat sink is positioned, or on a plane close to said plane.
 3. Amethod according to claim 1, wherein the screwing pieces are cutrespectively at positions close to edges of the seal member in such amanner that the through spaces for screwing do not remain.
 4. A methodaccording to claim 1, wherein the heat sink is formed of a thickmaterial difficult to deform, while the screwing pieces are formed of aflexible material easy to deform.
 5. A method according to claim 1,further comprising the steps of: positioning the first and second leadsin such a manner that the respective inner ends overlap the heat sinkinside the heat sink and spacedly a predetermined distance from sideedges of the heat sink; superimposing the first and second leads on theheat sink through insulating spacers and subsequently performing thewire bonding; covering the semiconductor chips and the wires with resinhigher in heat resistance than the resin which constitutes the sealmember, to form an undercoat resin; and in the one-side transfermolding, clamping and holding portions of the heat sink as well as thescrewing pieces and spacers superimposed on the heat sink, with use of amolding die, and forming the seal member which covers all of thesemiconductor chips, the wires and the undercoat resin.
 6. A methodaccording to claim 1, further comprising the step of: fixing asemiconductor chip with a field effect transistor formed thereon andsemiconductor chips with capacitors formed thereon to the main surfaceof the heat sink in such a manner the capacitor-formed semiconductorchips are positioned on both sides in the gate-drain direction of thefield effect transistor-formed semiconductor chip, wherein, in the wirebonding step, a drain electrode on an upper surface of the field effecttransistor and an upper electrode of one of the capacitor-formedsemiconductor chips are connected with each other through a plurality ofwires, and the upper electrode and the first lead are connected witheach other through a plurality of the wires, and wherein a gateelectrode on the upper surface of the field effect transistor and anupper electrode of the other capacitor-formed semiconductor chip areconnected with each other through a plurality of wires, and the upperelectrode and the second lead are connected with each other through aplurality of the wires, to constitute a high frequency power amplifierfor a base station.